Packaged light emitting diodes (LEDs), such as the XLamp® packaged light emitting diode manufactured by Cree, Inc., Durham, N.C., may include optical elements such as lenses arranged to focus and/or direct light emitted by a solid state light emitting device. Because of the high optical power and/or heat that is generated by a solid state light emitting device during operation, it may be desirable for the lens of such a packaged device to be capable of withstanding heat and/or light without degrading.
Small plastic elements may be manufactured through molding processes. However, molded plastic lenses having operating properties suitable for use in packaged LED applications have proven to be incapable of withstanding high temperatures and/or may degrade and lose light transmittance properties when exposed to light having a wavelength of 400 nm or less, which may be present in blue and/or near-UV light. Examples of plastics used for such lenses are PMMA (polymethyl methacrylate), COC (cyclic olefin co-polymer), polymethylpentene, and the like. Such plastic lenses may be formed, for example, through injection molding.
Silicone materials may possess desirable optical and thermal properties for a high performance packaged LED devices, since they may be capable of handling high heat and/or short wavelength light exposure without substantial degradation. However, silicone generally cannot be molded into optical shapes by injection molding as is the case with the thermal plastics mentioned above.
Some silicone materials have been developed that are capable of being molded by liquid injection molding and/or transfer molding. However, in both of these methods, a high percentage of material is wasted, as an excess of material is needed during each molding cycle to fill a cull, or receptacle, for the raw material, as well as a network of runners of the mold-die. During liquid injection molding or transfer molding, material is squeezed by a mold plunger in the cull area, forcing it to flow into runners and through gates that lead to the cavities that form the lens. Excess material in the cull area and the runners may be wasted, increasing the process costs.
In conventional molding techniques, the molded part may include excess material, referred to as “flashing,” around a circumference of the part where a seam between the die molds was formed. This flashing is typically removed from the molded part. Similarly, in transfer molding and injection molding techniques, material corresponding to the gate may be removed from the finished part.
The portion of the molded part around the excess material formed by the gate may have an increased level of material stress, which may become distorted when the temperature of the part is elevated. This may be particularly undesirable for an LED package lens since, as the lens heats up during LED operation, the lens may become distorted, which may change the near or far field optical pattern of the package. In addition, the portion of the part around the excess material may be roughened and/or otherwise damaged when the excess material is trimmed. Such distortion and/or damage may adversely affect the optical characteristics of a lens formed through transfer or injection molding, even if the gate is located near an edge of the molded part, and/or may complicate the assembly of the package.
Conventional transfer molding and injection molding techniques may have other drawbacks for molding silicone. For example, cured silicone may bond to metal and/or plastic surfaces. Thus, it may be difficult to use transfer molding to produce molded silicone parts, because the cured silicone may block the gate.
A third method of making a silicone lens is by potting. In this method, liquid resin is dispensed into a metal or plastic mold-cup and is then cured in an oven. This method may be labor intensive, slow and limited to forming incomplete shapes, as a portion of the lens so formed is defined by the liquid level of the curing resin.
A typical setup for injection/transfer molding of a lens is shown in FIG. 1. As shown therein, a mold 10 includes a plurality of lens-shaped cavities 12. Raw material provided in a feed chamber 14 is injected/transferred into the cavities 12 through a plurality of runners 16 and gates 18. The number of lenses that such a system may be capable of producing in a single run may be limited by the length and complexity of the system of runners 16. Furthermore, the lenses produced in such a system may include a gate due to the presence of the runners 16 that may be trimmed from the lenses before they may be used in a package. As noted above, the removal of the gate may result in a rough, uneven and/or stressed location on the resulting lens.